What is the right way to work 2 static 2d arrays in C?

Question

I have 2 static matrices. a[][N] and b[][M]. Both N and M are pre defined and are not equal to each other. Now how am I supposed to write functions that can recieve them both and work properly?

void PrintStaticMatrix(int matrix[][??], int rows, int cols);

What am I going to need to put insted of ??. The naive way is to seperate the function into two functions; they will be exactly the same, but one is for N and the other one is for M. Howeve, it does not seem to make any sense. Is not there a better way? I would have much perferred to use only dynamic arrays, but we are forced to in an advanced C course.

Vlad from Moscow · Accepted Answer · 2020-08-28T07:59:40.867

4

If your compiler supports variable length arrays you can declare the function like

void PrintStaticMatrix( int rows, int cols, int matrix[rows][cols] );

or

void PrintStaticMatrix( int rows, int cols, int matrix[][cols] );

or

void PrintStaticMatrix( int rows, int cols, int ( *matrix )[cols] );

Another approach is to use dynamically allocated arrays as for example

int **matrix = malloc( rows * sizeof( int * ) );
for ( int i = 0; i < rows; i++ )
{
    matrix[i] = malloc( cols * sizeof( int ) );
}

And declare the function like

void PrintStaticMatrix( int **matrix, int rows, int cols );

One more approach is reinterpret a two-dimensional array as a one-dimensional array. In this case the function will look like

void PrintStaticMatrix( int *matrix, int rows, int cols );

And a function call can look like

PrintStaticMatrix( ( int * )matrix, rows, cols );

Within the function you will deal with a one-dimensional array. Using an appropriate expression as an index you will be able to simulate a two-dimensional array for example in for loops.

Otherwise you will need two separate functions like

void PrintStaticMatrix1( int matrix[][M], int rows );

and

void PrintStaticMatrix2( int matrix[][N], int rows );

edited Aug 28 '20 at 07:59

answered Aug 27 '20 at 21:52

Vlad from Moscow

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The important thing is that the `cols` parameter has to be before the `matrix` parameter, right? – Barmar Aug 27 '20 at 21:53
@Barmar Otherwise you can can not specify the array. – Vlad from Moscow Aug 27 '20 at 21:54
My visual studio 2019 compiler does not allow it. What am I supposed to do? – Algo Aug 27 '20 at 21:54
@Algo It seems you are compiling the program as a C++ program. VLA is not a standard C++ feature. – Vlad from Moscow Aug 27 '20 at 21:55
Does VS support VLA? Is there a way to fix it? – Algo Aug 27 '20 at 21:56
@Algo Wait a minute. I will check MS VS 2019. – Vlad from Moscow Aug 27 '20 at 21:57
@Algo Yes. it seems MS VS 2019 does not support VLA. – Vlad from Moscow Aug 27 '20 at 22:09
Так что ты посоветуешь? – Algo Aug 27 '20 at 22:09
1

@Algo See my updated post. – Vlad from Moscow Aug 27 '20 at 22:14
@Algo in C++ you could use a template function. – Vlad from Moscow Aug 27 '20 at 22:26
@Algo See my appended answer. – Vlad from Moscow Aug 28 '20 at 07:59

What is the right way to work 2 static 2d arrays in C?

1 Answers1